The present invention relates to surface acoustic wave branching filters comprising band-pass filters having ladder constructions with surface acoustic wave resonators, in small-size mobile communication instruments such as portable telephones, particularly to surface acoustic wave branching filters usable even in case of high applied powers.
In recent years, developments of small-size, light mobile communication instruments such as portable telephones have been rapidly advanced. With this, reduction in size and improvement in performance of parts to be used have been pursued. To cope with this tendency, RF (Radio Frequency) parts using surface acoustic wave (hereinafter referred to as SAW) filters have been developed and used. In particular, SAW branching filters have been actively developed and part of them have been put in practical use and used because they are devices for largely contributing reduction in size of RF parts.
A SAW filter includes a transmission filter (hereinafter referred to as Tx filter), a reception filter (hereinafter referred to as Rx filter), and a branching line.
Since a high power is applied to the transmission filter, the transmission filter must be a SAW filter whose characteristics do not deteriorate even in the application of the high power. Such high-power SAW filters are disclosed in, e.g., Japanese Patent Application Laid-open Nos. 6-29779, 10-303682, and 11-251871.
As the filters used in the aforementioned prior arts, SAW filters are used in each of which the transverse length or the number of pairs of SAW resonators composing its transmission filter has been increased to increase the area per unit current and thereby raise its withstand power.
On the other hand, a SAW branching filter in a mobile communication terminal device such as a portable telephone has a function in which one antenna is used for both of transmission and reception.
Further, for this branching filter required is a function in which there is no change in characteristics in either of:
(1) a case that the antenna is in a normal operation, i.e., a case that the input impedance of the antenna is 50 Ω; and
(2) a case that the antenna is free, i.e., a case that the input impedance of the antenna is infinite.
In the above cases (1) and (2), because of a state of a rapid change in impedance, variation and break of SAW branching filter characteristics upon transmission in which a power is applied to the SAW branching filter, come into question. It is known that these variation and break of the SAW branching filter characteristics are caused by deterioration and break of a series-arm resonator of the transmission filter to which a high power is applied.